lh% ERTbBKATES 60

Int. Zoo Y6. (1991) 30: 60-67 The Zoological Society of London

Husbandry and moulting behaviour of the Robber or Coconut crab

at London Zoo Birgus latro

MATTHEW ROBERTSON Senior Keeper, Invertebrate Conservation Centre, The Zoological Society of London, Regent’s Park, London N W 1 4 R Y, Great Britain

The Robber or Coconut crab Birgus latro is the world’s largest land invertebrate. The genus contains only a single species and is placed in the family Coenobitidae together with the more familiar land hermit crabs which it resembles in appear- ance. However, except for a short period during its early post-larval (glaucothoal) stage (Reese & Kinzie, 1968), Birgus has completely lost the need for a protective gastropod shell.

Birgus is capable of attaining a great size, with reliable reports of specimens weighing up to 4 kg and having a leg span of over 75cm. A ‘shell width’ of over 60-90 cm (Vogel & Kent, 1970, 1971) has been reported but it seems unlikely that ‘shell width’ can be equated with ‘thoracic width’ in these cases, as such an individual would have a leg-span of over 270 cm and weigh in excess of 18 kg. We suggest that confusion has arisen over the terminology employed and that the most likely explanation is that ‘shell width’ refers to the leg span. It has been suggested that intensified predation has led to a general decrease in the size and, because of the long life-span that Birgus is believed to exhibit and the slow growth rate, it seems likely that this is true.

Birgus is found only on fairly remote islands and with one exception, that of the Japanese islands of the Ryu Kyu group, is restricted to the Indo-Pacific tropics. In the Indian Ocean it is found on three island groups, the Aldabran Atoll, Chagos Archipelago and the Nicobar and

Andaman Islands. In the western Pacific it is still found on many islands including the Solomon Islands, Guam, Caroline Island and Fiji. There is slight evidence to suggest that it might have occurred on the north-east Australian coast (Reyne, 1939) but none has been found on the mainland since this original observation.

The species is listed as Rare in the 1990 IUCN Red List of Threatened Animals (TUCN, 1990) and possibly the greatest threat to its survival is from human inter- vention, both direct and indirect. The crabs are harvested in large numbers as a food resource (Amesbury, 1980) as well as for their alleged medicinal properties. A further threat comes from the tourist trade for which large numbers are killed, stuffed with expanded foam and varnished for sale as curios (Wells et al., 1983). This practice occurs mainly in South-east Asia and in the western Pacific but the final products are shipped to many countries including mainland Australia. Unlike most species of crab, which are either aquatic or amphibious, Birgus usually inhabits inland scrub or forest up to 3 km from the sea and is therefore at risk from habitat destruction through agricultural (Sheppard, 1979) and other human activities.

Another threat is the introduction of various alien species into the crab’s habitat (Johnson, 1965; Daniel 8i Prem- Kumar, 1967). Among a number of feral species, the rat Rattus and the domestic pig Sus scorfa are recognized as posing

INVERTEBRATES 61

the greatest problem but monitor lizards Vuranus and monkeys have also been implicated (Linsley, 1934; Helfman, 1973; Kemm, 1982).

Several countries, including Guam, Tuvalu and Vanuatu, have introduced legislation restricting the harvesting of Birgus to specific areas, size of the indivi- dual and season but most countries have little or no management strategies for the species (Amesbury, 1980).

ACQUISITION BY LONDON ZOO The Invertebrate Section at London Zoo acquired 2.4 Birgus in August 1988 in order to study their husbandry and assess the possibilties of breeding them in cap- tivity. In 1989 a.further, injured, 6 was given into the Zoo’s care. The animals proved to be delicate and highly suscept- ible both to bacterial infection and disturbance, particularly during moulting. The 66 and three 99 died but one 9 has successfully completed moult on three occasions and at time of writing is living and apparently healthy.

The six animals originated from one of the coralline islands of the Aldabra Atoll and were flown to London from Mahk in the Seychelles, a three-day journey from Aldabra to the Zoo. In view of the crabs’ considerable strength, it was important that the transport containers were secure as well as comfortable. Specially constructed steel reinforced boxes, measuring 56 x 45 x 35 cm and divided in two by a steel diagonal division were used. The crabs travelled one to a compartment and all arrived in good condition. One 9 had lost its second pereiopod (first ambu- latory leg) on the right side prior to trans- portation but the injury appeared to have healed and there was no sign of infection.

A year later, on 16August 1989, a 6 was imported from the Solomon Islands apparently destined for the pet trade. After consultation with IUCN Conserva- tion Monitoring Centre at Cambridge, it was agreed that the animal should be deposited at the Zoo. Since this crab was from a completely separate population

there was no intention of its being used for breeding with the existing 99.

HOUSING The crabs are reported to be aggressive towards their own species (Helfman, 1977a) and, with the exception of two of the 99, they were housed individually, one in a display unit in the Insect House and the remainder off-exhibit. The display unit was constructed before the arrival of the Aldabran crabs based on information obtained from the literature (Altevogt & Davis, 1975; Hicks et al., 1984). The larger 6, which was considered almost certainly past the sexually active stage (Helfman, 1973), was chosen for exhibit. It was placed in the exhibit immediately on arrival but for the first four days the public window was covered with card until the crab appeared to be relaxed and was feeding regularly.

The second S. and two 99 were housed in individual plastic containers, measuring 60 x 100 x 70 cm deep, and the two smallest 99 (cephalthoracic length c. 70 mm) were placed together in one large plastic container, measuring 60 x 150 x 70 cm deep. The substrate was a 15 cm deep layer of washed silver sand kept permanently moist. At one end of the individual containers and each end of the shared container half a 36 cm ceramic plant pot (broken lengthways) was placed as a retreat beneath which the crabs built their burrows. In the display unit the burrow was formed from water-proof cement and was 30cm deep and 25cm high with a 10 cm layer of sand substrate. The entrance was partly concealed by a flange of concrete an arrangement which, although it restricted public viewing, was considered beneficial to the animal.

A large bowl of fresh water was supplied for drinking and bathing and helped to keep the relative humidity at c. 65% (range 63-75%). Initially the crabs were lightly sprayed with fresh water once a day to prevent desiccation (Harms, 1932). The containers had originally been left open for ventilation and when they

62 IhVERTEBRATES

were later fitted with loose polythene lids to reduce disturbance, the humidity values rose to 90% (79-100%) and spraying was stopped. Although Birgus is found several kilometres from the sea and there is no firm evidence that they need to drink salt water (Gross, 1955), natural sea water was provided during the first two weeks as we were concerned that homeostasis of the ion concentration of the haemolymph (700-800 mOsrn.kg-'H,O, Greenaway, 1988) might have been disturbed during transport. However, it is unlikely that any serious imbalance had occurred as it would appear that Birgus, like a number of other land crabs (Wolcott & Wolcott, 1982, 1985), is able to reprocess its urine by passing it into the branchial chamber. On the first day it was offered, some of the crabs appeared to drink the salt water but by the end of the week they showed no further interest. After the initial period, fresh water was considered sufficient to keep the crabs in good health (Gross, 1964; Harris & Kormanik, 1981).

During the first year the off-exhibit animals were housed in a converted section of the tropical species' rearing room. However, we had already been concerned about disturbance and stress levels and, when one of the 66 died during moult in July 1989, they were moved to a smaller, more remote room where it was easier to control the tempera- ture and there was less human traffic.

In the rearing room the crabs were kept at a mean temperature of 24°C (23-31°C) but after the move this was changed to a day/night schedule of 1 1 hours light: 13 hours dark at c. 28°C (27-29°C) reduced to 24°C (23-26°C) at night, a regime based on the climate of the Aldabran Atoll (Alexander, 1976). Burrow tempera- tures, checked before the arrival of the crabs, remained fairly constant at 25- 26°C with a relative humidity of 93%. Temperature probes were permanently in place inside the containers but once the crabs had taken up residence, no attempt was made to record burrow temperatures as any probe would undoubtedly have

been rapidly dismantled by these inquis- tive animals or, as occurred with probe which was accidentally allowed to hang near the floor of a container, used as a convenient climbing rope. Lighting was provided initially by a 60 W fluorescent tube but in the new quarters this was replaced by a 150 W incandescent light bulb. Some natural daylight was also present.

A careful watch was kept on the two 90 housed together but no sign of aggressive behaviour was observed; they were often seen in close proximity and would even feed from the same dish. This appears to contradict nearly all the observations of Birgus in the wild. In describing their agonistic behaviour Helfman (1977a, 1979) refers to serious maiming and even death resulting from chance encounters. The lack of aggression in this case might be attributable to a number of factors, including that the crabs were of equal size and the same sex, and that food was plentiful.

FEEDING We offered a variety of foods based on observations in the wild and in the labora- tory (Reyne, 1939; Gibson-Hill, 1947; Grubb, 1971; Alexander, 19763. Coconuts and freshly killed mice were readily accepted and fish and a variety of fruits, including apple, orange, banana and pine- apple, were also taken. They also accepted shore crabs but because of the risk of cross-infection (Krantz, et al., 1969; Legise & Raguenes, 1975) these were removed from the diet. All plant and animal food items were thoroughly washed and fed to the crabs on dishes. Fresh drinking water was always avail- able. The crabs received some calcium from the bones of the mice but cuttlebone was also available as a mineral supple- ment and was readily consumed, the crabs scraping off small amounts with their chelae. A fish-based meat diet, ZF6, was also accepted but was discontinued since the crabs smeared the soft food into the

INVERTEBRATES 63

sand producing an ideal medium for pathogens.

When first placed in the containers the crabs were active at all times of the day, although most of the feeding took place at night. However, within a few weeks they had adopted a more nocturnal habit. Activity outside the burrow could be judged by the amount of disturbance to the substrate and the amount of food eaten each morning. Most of the items offered were eaten in relatively large quantities at the beginning but the crabs gradually became more selective in their food preferences. Although most foods would be tried, apart from freshly killed mice and to a lesser extent coconut, only small amounts were consumed.

On 5March 1989 after six months of routine maintenance the 8 on exhibit became lethargic and two days later died. No post-mortem examination was carried out and no cause of death established, although old age could have been a con- tributory factor.

MITES Although every effort had been made to keep the introduction of harmful agents to a minimum, the warm, humid condi- tions were ideal breeding grounds for a variety of pathogens. Two months after the original animals’ arrival, mites were observed on the sides of the containers and when the animals were examined closely numerous mites were found on the pleural membrane between the abdominal somites and on the membrane at the base of the coxa on most ambulatory legs. They were identified as Caloglyphus, frugivorous mites which were thought to pose little direct threat to the health of the crabs but there was a danger that in large numbers they could cause damage to the sensitive ‘lungs’ or other delicate mem- branes. All contaminated materials were sterilized or discarded and large numbers of mites were removed mechanically with a water sprayer. This operation has to be performed with great care not only because it is stressful to the

animals but also because of the possibility a of injury to the keeper holding the crab.

BACTERIAL INFECTIONS A thorough examination of the Solomon Islands’ crab on its arrival at the Zoo revealed a number of injuries. The most serious of which was a 1 cm hole on the right side of the thorax, apparently caused, possibly during transport, by a nail or similar sharp object. The tough exoskeleton had been penetrated and the object had caused damage to the respira- tory tissue and possibly to the lung artery. The damaged area was covered with a sterile dressing but after a few days the hole appeared to have enlarged and the crab was treated with a topical anti- bacterial by the veterinary staff for what appeared to be an infection. The crab also showed signs of malnutrition; its abdomen was shrunken and the abdominal somites along the top of the abdomen were overlapping.

Because of shortage of space the new arrival was housed in the same room as the remaining crabs. To minimize the possibility of cross-infection there was no direct contact between the crabs and in- direct contact via the keepers was avoided wherever possible. The crab was housed and fed exactly as were the original animals but showed far greater activity levels and ate almost everything that was offered.

A little less than a month after arrival the animal became less active and on 13 September it died. The original hole had almost doubled in size and a second hole had started to appear on the opposite side of the thorax. On post-mortem exam- ination the lobulate respiratory tissue was found to contain necrotic patches and these had spread into the lungs. On bacteriology, a pure culture of Vibrio parahaemolyticus was isolated from the heart and this was probably a significant finding; this bacterium is a known patho- gen of custaceans and other animals (Krantz et al., 1969; Johnson, 1976). To our knowledge V . parahaemolyticus infec-

64 INCtRTEBR4TES

tions occur in highly concentrated condi- tions, such as captive groups, and we know of no reports of serious outbreaks in wild crustacean populations. This suggests that stress and proximity play a large part in deaths attributed to the bac- terium. The haemocoelic infection it causes adversely affects moulting crabs of other species, either by extending the intermoult period or by preventing the moult from being completed successfully (Johnson, 1976).

Although the original injury to this crab’s thorax was probably mechanical in origin, V. parahaemolyticus is a chitino- lytic bacterium and probably caused the growth of the holes. In an attempt to prevent the spread of the infection all the remaining crabs were rehoused and materials were sterilized or replaced.

Despite these precautions, on 17 September one of the 99 became leth- argic, ceased to feed and died the next day. Vibrio parahaemolyticus, was isolated from several tissues, including the heart.

Vibrio parahaemolyticus was probably introduced into the collection via the Solomon Islands’ crab and the experience underlines the importance of a quarantine period for all animals newly imported into a collection. Unfortunately, as specific external symptoms cannot be observed until the infection has reached an advanced state, providing an effective quarantine system for this particular pathogen is difficult. The extraction of body fluids would cause serious stress to the crab and the animals rarely produce faeces.

One 9 had died on 5September, after the arrival of the Solomon Islands’ crab but before the above two deaths. There had been no warning signs and no pre- moult behaviour had been observed. Although it was not detected, V.para- haemolyticus may also have been impli- cated in the death of this individual.

MOULT Two crabs died during moult; a third, the surviving 9, had moulted successfully on

three occasions at time of writing. The second Aldabran 8 moulted on 2 July 1989. The animal made no attempt to bury itself (Reyne, 1939; Johnson, 1965; Alexander, 1976) but ceased feeding and became lethargic three days before it began to shed its exoskeleton. After the moult was completed the crab remained completely still, resting on its exuvium. Seven days later it began to feed on the discarded shell but consumed only a small portion and after four days the activity ceased. The crab made no further attempt to feed and the exoskeleton remained soft and pale blue in colour. On 21 July, 19 days after moulting, it was found dead in the burrow. No post-mortem examination was carried out and there was no obvious cause of death but, because of the high level of human activity in the vicinity of the crabs, the possibility that stress had had an adverse effect during the vulner- able stage could not be ruled out, lead- ing to the rehousing of the surviving crabs.

On 14August one of the 99 became lethargic and ceased feeding. Its abdomen was large and it held it extended behind, as previously observed in a young crab prior to moulting (Held, 1963). Studies of wild crabs have shown that prior to moulting large quantities of fat are present in the abdomen and probably act as a food reservoir. We avoided disturbing the animal but on the morning of 20August it was found dead in its burrow. The post-mortem examination revealed that the abdomen was packed with fat indicating that it was probably in a pre-moult phase. The bacterium Aero- monas sobria, which has been recorded as causing large numbers of fatalities in crabs in their moulting stage (Legise & Raguenes, 1975), was isolated.

A clear orange jelly-like substance found towards the rear of the crab’s thorax, close to its ovaries, is believed to have been developing embryonic material. As spawning takes place between the months of January and March, the material was at a very early stage of

INVERTEBRATES 65

Plate 1. A freshly moulted adult 9 Robber crab Sirgus fafro at London Zoo. D. Clarke.

development and no structure could be observed.

At the same time another 9 began to show a marked decrease in activity. Its abdomen had steadily enlarged over the previous 12 days until it was held extended behind and on 20August the crab retreated into its burrow. It made only a cursory attempt to bury itself, forming a shallow depression 19cm diameter and 7 cm deep in the sand. Three days later it moulted and by the 27th the exoskeleton had hardened and the crab became increasingly active. The shell was blue in colour with a bright spot at the centre of the thorax (Plate 1). The thor- acic width had increased by only 2mm from 88-90mm. Within eight days the crab was feeding avidly and beginning to increase in weight. An inspection of the burrow showed that almost none of its exuvium remained.

This 0 continued to feed well until the 13January 1990, when it began to dig another entrance to its burrow; two days later it had completely buried itself and

sealed both entrances. It was left undis- turbed, although movement could be heard inside the burrow until the middle of February, probably the point at which the moult was achieved. This was some 178 days after the last moult. On 22 March, after 67 days sealed within the burrow, the crab showed signs of emerging. An endoscope was pushed a few centimetres into the burrow but was immediately withdrawn when the crab, which still had its post-moult blue colour, instantly lifted its second pereiopod in an aggressive ambulatory raise and presented both chela, leaving us in no doubt of its aggressive intention. Five days later it was offered a mouse and the following morning had eaten all but the tail. Its activity levels began to increase.

As the remains of previous meals were a possible health hazard, on 3 April, the crab was moved to a clean container. An inspection of the burrow revealed only very small pieces of exuvium. When the animal was examined the cephalothorax was found to be slightly soft but its exo-

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skeleton hardened gradually over an eight- day period during which its appetite increased. After a week it settled into typical inter-moult behaviour until the 4 September when its behaviour became markedly erratic. Feeding intensified and it was found outside the burrow at all times during the day until 7 September when it again sealed itself into the burrow.

Activity within the burrow ceased in early November. The crab spent a total of 91 days in the burrow and emerged on 6December. However, the day before it was seen to leave the burrow it was offered a mouse and this was completely eaten. The next day there were signs that it had been to its water bowl. While it was out of the burrow it was measured but no increase in size was recorded and the thor- acic width remained 90 mm. It is therefore assumed that this 0 has attained full size.

CONCLUSION Despite their robust appearance, Birgus are delicate animals prone to a number of problems when kept in captivity. If these fascinating creatures are to be kept successfully great care needs to be taken to avoid subjecting them to even moderate levels of stress especially prior to moulting. In the past this seems to have been a major problem with the main- tenance of mature crabs (total cephalo- thoracic length greater than 6 cm).

So far only a few attempts have been made to rear the larval stage of Birgus and none has been successful. However, all were made in the field and poor rearing facilities probably contributed to the failure. One of the problems seems to be making sure that the post-larval crabs do not drown and that they adopt a molluscan shell. There is only one report of a mating which was observed taking place on land (Helfman, 1977b). This contradicts the mating behaviour predicted by Matthews (1 956) which assumes that this would take place in the sea as in all other coenobitids. If the animal is to be kept in zoos and aquaria, it is essential that its reproduction in cap-

tivity is mastered. However, as our investigation has shown, there is still much to be learned about the require- ments of Sirgus latro in captivity.

ACKNOWLEDGEMENTS

Thanks to Rod Hall, Principal Coordinator and Development Officer Nature Conservation Projects, British Airways; Mr Andre of the Seychelles Wildlife Department; Ray Ingal, Head of Crustacean Department, British Museum (Natural History) for their help in this project. We are grateful to Andrew Cunningham, Veterinary Pathologist, for his advice on that part of the text dealing with pathology and to him and Martine Carole for pathological examination of the crabs.

PRODUCT MENTIONED IN THE TEXT

Z o o F d 6 (ZF6): manufactured for London Zoo by Spillers Foods Ltd, New Malden House, New Malden, Surrey KT3 4TB, Great Britain.

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Manuscript submitted 15 February 1991